These differ from each other with the amount of information we can find on a given device: e.g. User manual Casio CTK-401 is a certain type of technical documentation being an integral element of any device we purchase. Below are few guidelines regarding how and why you should collect the product manuals. However, you are not the only person having problems with keeping the operation manual of all household devices. User manual Casio CTK-401 - have you ever lost it? If you are here, it was rather the case.